DETERMINATION OF CONTACT ANGLES AND PORE SIZES OF POROUS-MEDIA BY COLUMN AND THIN-LAYER WICKING

A simple method is described for the determination of contact angles (theta) on powdered materials such as clay particles. This is done by depositing the particles from a liquid suspension onto glass slides, by sedimentation, followed by drying. The dried thin-layer plates are then subjected to wicking in a number of liquids using the Washburn equation to determine cos-theta. However, one other unknown in the Washburn equation, i.e. the average interstitial pore radius R, must first be determined. This is done by wicking with low-energy spreading liquids, such as alkanes. It could be shown with spherical monosized polymer particles, as well as with clay particles, that spreading liquids pre-wet the surfaces of the particles over which they subsequently spread. Thus, it can be demonstrated that spreading coefficients, in the sense of Harkins, play no role in this type of spreading and cos-theta equals unity in the Washburn equation for all values of gamma(L), for all spreading liquids(L). Results were obtained by thin layer wicking, yielding the surface tension components and parameters for the clays talc and illite. These results also indicate that with non-spreading, apolar as well as polar liquids (including water), no spreading pressure occurs on the materials tested.